The present invention relates generally to infrared (IR) cameras, and more specifically the invention pertains to a histogram projection system which automatically optimizes, tracks changes in luminance and adjusts in real time the display of wide dynamic range imagery from IR cameras.
The task of providing a histogram projection system which automatically tracks changes in luminance to adjust the display of wide dynamic range IR imagery is alleviated, to some extent, by the systems disclosed in the following U.S. patents, the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 4,656,665 issued to Pennebaker;
U.S. Pat. No. 4,670,653 issued to McConkel;
U.S. Pat. No. 4,719,350 issued to Alm;
U.S. Pat. No. 4,741,046 issued to Matsunawa; and
U.S. Pat. No. 4,856,528 issued to Yang.
The patent to Pennebaker describes a system for thresholding images to reduce the information content using histogram analysis. The patent to McConkle teaches an infrared detection and imaging system. The patent to Alm describes a radiation imaging enhancement system employing a feedback loop to obtain an average value detection signal and a reference signal. The patent to Matsunawa teaches a histograming method discriminating picture wherein the picture is binary coded. The patent to Yang teaches a histograming method in a CT image in medical applications.
It is common for IR cameras to rely on manual gain and offset adjustment to obtain a real-time mapping of IR signals to the display values. The manual adjustment gives an excellent view in general of the useful IR information in a scene, but it requires frequent readjustment as the camera is panned or if the IR luminance changes (sun goes behind a cloud for example). Moreover as a linear process, the manual offset/gain expends, so to speak, useful dynamic range on empty levels within the histogram or information range of the IR levels. Two alternative candidates, which we have considered, for implementing automatic adjustment are direct scaling and histogram equalization.
Direct scaling (also called "black level and gain") requires that the histogram of each frame be analyzed to set a "black" and "white" level. The optimum way to do this is scene and application dependent and requires either a user interactive parameter in the implementation or a plethora of choices. In either case, more complexity is involved than with the new algorithm described below. Further, as with the manual adjustment to which it is closely analogous, one is still faced with the disadvantage cited above for a linear process.
The familiar technique, described in text books, of "histogram equalization" assigns dynamic range to each occupied level in proportion to how many pixels are at that level. Extensive simulations in software on actual imagery: indoor and outdoor; day and night, show that due to the nature of IR histograms, histogram equalization amplifies shot noise and loses gray scale resolution for smaller (usually hotter) objects in the scene. The purpose of the present invention is to avoid this pitfall.